The invention relates to adhesion promoters for the production of vulcanizates obtained from heat-vulcanizable compositions and exhibiting a favorable filler/elastomer bond.
Such compositions are characterized by the following features:
They comprise an active, reinforcing filler, an elastomer component, an effective amount of a vulcanizing agent, and conventional additives
The filler comprises a highly disperse filler from the group of silicic acids, silicates and mixtures thereof, and optionally rubber carbon blacks.
The elastomer component consists of one or several olefinically unsaturated elastomers capable of vulcanization with the vulcanizing agent.
The vulcanizing agent is a vulcanization system containing sulfur or sulfur donors in combination with conventional accelerators.
Heat-vulcanizable compositions are generally obtained by means of a process characterized by the following features:
A basic mixture (preliminary batch) is prepared in the first stage by hot mixing of the components, except for the vulcanizing agent, at higher than 130.degree. C. in a kneader.
In the second stage, the vulcanizing agent is admixed (warm mixing) at lower than 100.degree. C., especially lower than/equal to 70.degree. C., i.e. generally far below the initiating temperature of the vulcanizing agent, for example on a roll.
In order to achieve a favorable filler/elastomer bond in the vulcanizates, it has proved necessary to treat the silicic acids and silicates with adhesion promoters. These must exhibit, on the one hand, groups capable of forming a stable chemical bond with the filler surface (typically a siloxane bond). On the other hand, they must exhibit groups capable of forming, during vulcanization, a stable chemical bond with the elastomer component (sulfur bridges) Accordingly, these adhesion promoters make possible the manufacture of vulcanizates exhibiting a favorable filler/elastomer bond.
The adhesion promoter is normally admixed in the first process stage. It can also be added in the second process stage prior to addition of the vulcanizing agent.
A conventional adhesion promoter is gamma-mercaptopropyltriethoxysilane. One disadvantage of said promoter resides in its volatility and unpleasant odor in handling. Another disadvantage is the premature occurrence of vulcanization, which may result in an impairment of flow characteristics and even in scorch, during processing of vulcanizable compositions containing a filler treated with this adhesion promoter.
Therefore, the practice has been adopted of using adhesion promoters overcoming these disadvantages characterized by the following formula: EQU (C.sub.2 H.sub.5 O).sub.3 Si--CH.sub.2 CH.sub.2 CH.sub.2 --S.sub.x --CH.sub.2 CH.sub.2 --CH.sub.2 --Si(OC.sub.2 H.sub.5).sub.3
wherein x is the number of S atoms in the individual compounds. The adhesion promoter is an oligosulfane (oligosulfide) which is characterized by a value V which is the number of S atoms, based on two Si atoms, as determined by elementary analyses (German Patent No. 2,712,866). In a commercially available adhesion promoter, V is approximately 4 (a "tetrasulfane").
Vulcanizable compositions obtained with the state of the art adhesion promoters show, in individual cases, a significant reduction in vulcanizing speed as compared with a vulcanizable composition obtained without an adhesion promoter.